In a cellular communication, a user equipment (UE) accesses a base station to perform communication, receives control information for transceiving data from the base station and transceives data with the base station. In particular, since the UE transmits and receives data through the base station, the UE transmits data of the UE to the base station in order to transmit the data to a different cellular UE. Having received the data, the base station transmits the data to a different UE. As mentioned above, since a UE is able to transmit data to a different UE via a base station only, the base station performs scheduling on a channel and a resource for transceiving the data and transmits scheduling information on the channel and the resource to each UE. As mentioned in the foregoing description, in order to perform D2D (device-to-device) communication via the base station, it is necessary for each UE to have a channel and a resource allocated by the base station to transceive the data. Yet, the D2D communication has a structure that a UE directly transceives a signal with a UE intending to transmit data without passing through a base station or a relay.
In case of performing the D2D communication directly transceiving data between UEs in a manner of sharing a resource with a legacy cellular network, each UE performs the D2D communication after a resource for the D2D communication is allocated. However, a method of transmitting an initial signal for initiating the D2D communication has not been defined yet. And, in case of performing the D2D communication directly transceiving data between UEs in a manner of sharing a resource with a legacy cellular network, synchronization of a UE performing the D2D communication and synchronization of a UE communicating with the cellular network may not be matched with each other. And, synchronization of UEs different from each other and synchronization of UEs performing the D2D communication may not be matched with each other.